TITLE:   GCN CIRCULAR
NUMBER:  30485
SUBJECT: GRB 210723A: Swift detection of a burst
DATE:    21/07/23 15:04:47 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

T. Sbarrato (INAF-OAB), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), N. P. M. Kuin (UCL-MSSL),
S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 14:46:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210723A (trigger=1061284).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 121.726, -32.889 which is 
   RA(J2000) = 08h 06m 54s
   Dec(J2000) = -32d 53' 20"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 50 sec.  The peak count rate
was ~2200 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 14:47:40.4 UT, 89.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 121.72157,
-32.88624 which is equivalent to:
   RA(J2000)  = 08h 06m 53.18s
   Dec(J2000) = -32d 53' 10.5"
with an uncertainty of 7.0 arcseconds (radius, 90% containment). This
position may be improved as more data are received; the latest position
is available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 7.70
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 3.04e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 99 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the large, but uncertain, extinction
expected. 

Although the data from this burst are consistent with a classical long
GRB, we note that this source is close to the Galactic plane 
(lat = -0.34 degrees) which raises the possibility that this is
a Galactic transient.  If further analysis determines that to be
the case, we would name the object Swift J0806.9-3253 . 

Burst Advocate for this burst is T. Sbarrato (tullia.sbarrato AT inaf.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)